The present invention relates to a coaxial waveguide assembly with an air-tight window plate made of a dielectric material and, more particularly, to an output section of a microwave tube such as a klystron.
An output section of a conventional microwave tube such as a klystron is exemplified by a structure wherein a coaxial waveguide is connected to an output reasonant cavity and a rectangular waveguide is connected to the distal end of the coaxial waveguide. An air-tight window plate made of a dielectric ceramic material is air-tightly mounted on the distal end of the coaxial waveguide assembly. A typical arrangement of a straight beam multicavity klystron having the above coaxial waveguide is shown in FIG. 1. As shown in FIG. 1, the klystron body comprises intermediate resonant cavity 11, drift tube 12, output cavity 13, and collector section 14, all of which are connected in tandem with each other along the axial direction of the klystron. Coaxial waveguide assembly 15 serving as an output section is air-tightly connected to part of the cavity wall of output cavity 13. Assembly 15 comprises inner and outer conductors 16 and 17. Cooling water circulates in inner conductor 16 in a direction indicated by arrow Q. Similarly, cooling water circulates in outer conductor 17. Conductors 16 and 17 comprise large-diameter sections 18 and 19 formed such that the diameters of conductors 16 and 17 are increased midway thereof, respectively. Dielectric air-tight window plate 20 is air-tightly joined between the large-diameter sections 18 and 19. Large-diameter sections 18 and 19 have joint sections 18a and 19a at which conductors 17 and 18 are separated into halves along their axial direction. Joint sections 18a and 19a are located near the output resonance cavity with respect to air-tight window plate 20. Joint sections 18a and 19a are obtained by electrically connecting the halves into integral air-tight assembly by arc welding or the like. The distal end of coaxial waveguide 15 is connected to rectangular waveguide 21. More specifically, the distal end flange of large-diameter section 19 of the outer conductor is connected to an opening of wide surface 22 of rectangular waveguide 21. Distal end 18b of large-diameter section 18 is electrically and mechanically connected to wide surface 24 of rectangular waveguide 21 through cup-like enlarged section 23. It should be noted that opening flange 25 of the output waveguide is connected to an external RF load circuit.
In particular, conventional coaxial waveguide assemblies for high-power applications employ structures wherein air-tight joint sections are externally cooled in order to protect the internal and external joint sections of the air-tight window plates made of a dielectric material. Furthermore, in order to prevent the dielectric air-tight window plate from being damaged by multipactor discharge, a coating layer for suppressing the multipactor discharge is formed on the inner surface of the window plate. For this reason, in assembly of the coaxial waveguide assembly, the inner and outer conductor parts joined to the air-tight window plate are prepared separately from other parts such as inner and outer conductor sections 16, 17 connected to the resonance cavity. In the final stage of assembly, they are joined together to constitute an integral klystron assembly. In a conventional klystron, the inner and outer conductors are joined together by welding.
However, if welded parts of the inner and outer conductors are present as parts through which an RF current flows, these welded parts are undesirably heated. In order to prevent heat generation, a metal material (e.g., copper) having a high conductivity is used as a welding material for the welded parts. However, reliability of the welded parts is often degraded undesirably due to the properties of the welding material. Moreover, if the inner conductor must be welded within the outer conductor assembly in which the air-tight window plate made is not air-tightly welded thereto, it is very difficult to perform welding due to the presence of the outer conductor. In such a conventional structure, high reliability of the integral parts of the inner and outer conductors in the coaxial waveguide cannot be satisfactorily obtained.